The present invention relates generally to electrical discharge machining (EDM), and, more specifically, to an improved electrode for forming high aspect ratio holes.
Electrical discharge machining is a common process for forming holes of various shapes and configurations in a metal workpiece. An electrode having the desired shape is advanced toward the workpiece, with a suitable DC power supply effecting an anode and cathode between the workpiece and electrode for forming a controlled spark which melts and vaporizes the workpiece material to form the desired hole. An insulating liquid or dielectric fluid, such as a suitable oil or de-ionized water, provides insulation against premature spark discharge, cools the machined area, and flushes away the removed material.
EDM is commonly used for forming cooling air holes or slots in gas turbine airfoils to provide film cooling thereof during operation. Large turbine airfoils are used in an exemplary land based gas turbine engine sized and configured for power generation, such as driving an electrical generator in a utility grid. Pressurized air from a compressor is mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream to a turbine which powers the compressor as well as produces output power. A typical turbine stage includes a row of nozzle vanes and a corresponding row of rotor blades mounted to the perimeter of a rotor disk. The vanes and blades have suitably configured airfoils over which the hot combustion gases flow for extracting energy therefrom. The vanes and blades are therefore typically cooled using a portion of the pressurized air bled from the compressor which is channeled through the respective airfoils and out various holes formed through the walls thereof.
Vane and blade airfoils conventionally include leading and trailing edges extending from root to tip, with opposite concave and convex sides forming respective pressure and suction surfaces. The inside of the airfoil is hollow and is suitably provided with the pressurized cooling air which is discharged through various types of film cooling holes extending through either or both of the pressure and suction sidewalls. In one typical embodiment, the film cooling holes are inclined at a small acute angle of about 15.degree. to the surface of the sidewall and each extends through a considerable axial length of the sidewall for defining an inlet inside the airfoil and an outlet on the outside surface. The cooling air is channeled from the inside of the airfoil, out the film cooling holes, and develops a boundary layer of film cooling air for protecting the airfoil from the hot environment created by the combustion gases.
Typical film cooling holes are usually round in cross section and may be formed by conventional EDM or laser drilling for example. Non-circular film cooling holes, such as rectangular holes, have been developed for improving film cooling effectiveness. Rectangular holes or slots have a section aspect ratio defined by the longest side or height divided by the shortest side or width which is preferably greater than 1.0. The holes also have a depth aspect ratio defined by the length or depth of the hole through the airfoil sidewall divided by the width, which is typically larger than the section aspect ratio. In the large turbine airfoils found in a power generation gas turbine engine, the depth aspect ratio can be on the order of about 50, for example with a two inch length and a 40 mil width.
Accordingly, the large section and depth aspect ratios of an exemplary rectangular film cooling hole or slot at the small inclination angle of about 15.degree. substantially increases the complexity of forming such holes during the manufacturing of the vane or blade airfoils.
For example, conventional EDM employs either solid or flush-through hollow electrodes for producing holes in a workpiece wall such as a turbine airfoil. The solid electrode may be formed with the required shape, and the dielectric fluid is suitably channeled between the electrode and the hole being formed thereby. In the flushing electrode, a center passage is provided through the electrode for conventionally providing the dielectric fluid through the electrode so that deeper or higher depth aspect ratio holes may be formed. The solid electrodes are limited in machining depths by their ability to effectively remove material debris during the machining process, with flushing electrodes being used for machining greater hole depths since they more effectively flush the dielectric fluid.
Since EDM electrodes are consumed during operation, the turnaround time for manufacturing electrodes and the cost thereof are significant factors in the electrode design. In order to inexpensively produce flushing electrodes, circular tubes are typically plastically flattened into a generally rectangular or square configuration for forming corresponding rectangular or square EDM holes in the workpiece. However, there are practical limits in providing these flattened flushing electrodes with relatively large section aspect ratios. If the aspect ratio is too large, the walls of the tube during the manufacturing process will collapse into a dogbone shape and lose the rectangular shape desired.
Furthermore, at relatively large section aspect ratios, the long sides of the rectangular become more resilient which can destabilize the EDM process due to dynamic vibratory excitation of the long walls, which in turn can cause undesirable hole shape or oversizing thereof. Electrode deflection at the shallow entry angles, of about 15.degree. for example, can also occur which decreases the accuracy of the hole location along the minor axis of the rectangle, as well as causing inefficient and slow EDM cutting action.
The wall thickness of the tubular electrode must be suitably large for both accommodating inherent wear of the electrode during the EDM process as well as providing suitable structural stiffness of the electrode. The required center passage in the flushing electrode provides a practical limit on the minimum dimension of the electrode. In view of these exemplary considerations, a conventional, rectangular flushing electrode has an upper section aspect ratio of about 1.6 for forming a rectangular hole of about 80 by 50 mils for example. Since higher section aspect ratios substantially greater than this presently attainable ratio are desired for improving film cooling slot performance, an improved EDM electrode is desired.